Hitherto, an optical recording medium and a magnetic recording medium have been widely known as a circular disk form recording medium such as CD, CD-ROM, DVD, DVR, MD and the like on which audio, video and other various kinds of information, a servo signal and the like are recorded. These recording media include a phase change type optical disk such that a laser beam is radiated onto a synthetic resin disk substrate on which such signals as information signals and a tracking servo signal are written in the form of pits and a groove (guide groove) and the signals are read by utilizing a change in reflectance due to a change of crystal structure of a recording layer, an opto-magnetic disk for reading signals by utilizing a magneto-optical effect, a magnetic disk for writing and reading signals magnetically, and so on.
As a method for forming information signals, a tracking servo signal and the like in the form of fine roughness such as pits, a groove and the like in the recording layer of the disk substrate, a method of injection molding the disk substrate by use of a mold apparatus is generally used today.
FIGS. 22 to 24 show a mold apparatus 51 of a projected form gate cutting system using a fixed side stamper according to the prior art, in which a cavity 54 as a circular disk form space is vertically formed between faying surfaces of a fixed die 52 and a movable die 53. A stamper 55 is vertically disposed on the fixed die 52 side of the cavity 54, and the innermost circumference of the stamper 55 is fixed to a fixed mirror surface by a mechanical clamp. A cylindrical sprue bushing 56 is horizontally disposed in the fixed die 51 at a central portion of the cavity 54, and a cylindrical projected form gate cutter (also called “punch”), a small diameter ejector pin 58 and a cylindrical ejector 59 are horizontally disposed at a position opposite to the sprue bushing 56. The ejector pin 58 is disposed at the center of the projected form gate cutter 57, and the ejector 59 is disposed at the outer circumference of the projected form gate cutter 57.
A sprue 60 at the center of the sprue bushing 56 to which an injection cylinder (not shown) is connected is opened at the center of a projected form gate forming recessed portion 61 formed at the tip of the sprue bushing 56, and the tip of the projected form gate cutter 57 is formed at a projected form gate forming projected portion 62. A projected form gate 64 formed in a projected shape relative to a signal transfer side surface 63 which is a surface on the stamper 55 side of the cavity 54 is formed between the recessed portion 61 and the projected portion 62. Therefore, the projected form gate cutter 57 is a projected form gate cutter for forming the projected form gate 64.
In the mold apparatus 51 of the projected form gate system according to the prior art, a molten resin P1 consisting of a plasticized polycarbonate or other synthetic resin is injected in the direction of arrow a from the injection cylinder into the sprue 60 and is charged under pressure into the cavity 54 through the projected form gate 64, in the condition where the fixed die 52 and the movable die 53 are heated. In this case, the molten resin P1 compressed to a high pressure by the injection cylinder is pressurized onto the fine roughness surface of the stamper 55, whereby a disk substrate 73 in which signals 71 such as information signals, a tracking servo signal and the like are transferred onto a signal transfer surface 72 in the form of pits, a groove and the like is injection molded, as shown in FIGS. 25 and 26. Thereafter, a center hole 74 of the disk substrate 73 is formed by punching.
In this case, the conditions of accuracy of transfer of the signals 71 onto the disk substrate 73 are determined primarily by the plasticized molten resin temperature, the mold temperature, and the injection pressure of the injection cylinder, whereas the warpage and the like of the disk substrate 73 thus injection molded are determined by the mold temperature, injection pressure and cooling time.
The formation of the center hole 74 of the disk substrate 73 injection molded is generally carried out during the process of cooling the fixed die 52 and the movable die 53 while continuing the compression of the molten resin P1 charged in the cavity 54.
Hitherto, the center hole 74 as a circular hole has been formed in the center of the disk substrate 73 by punching (called “gate cutting”), by a method in which the projected form gate cutter 57 is projected in the direction of arrow b from a retracted position shown in FIG. 23 to an advanced position shown in FIG. 24 so as to cut the incompletely solidified resin between an outer circumferential surface 62a of the projected portion 62 of the projected form gate cutter 57 and an inner circumferential surface 61a of the recessed portion 61 of the sprue bushing 56. At this time, a roughly T-shaped sprue and gate remaining resin 73a remaining in the sprue 60 and the projected form gate 64 is ejected in the direction of arrow b from the signal transfer surface 72 of the disk substrate 73 toward the fixed die 52 side.
As shown in FIG. 25, the center hole 74 of the optical disk or the like of 12 cm in diameter, such as CD, CD-ROM, DVD, DVR, etc., has a diameter φ=15.0 mm, whereas the center hole 74 of an MD or the like has a diameter φ=11.0 mm.
As shown in FIG. 25, the center hole 74 thus formed is formed as a straight hole of which the hole diameter is parallel to the axial direction over the entire thickness of the disk substrate 73.
Though the timing of the punching of the center hole 74 varies according to the kind of the synthetic resin or the like, the punching is preferably conducted by the projected form gate cutter 57 before the molten resin P1 is completely solidified, and is said to be preferably conducted within a period of about 2 sec after completion of the injection of the molten resin P1. When the timing of the punching of the center hole 74 is delayed from the above-mentioned, strain due to punching and punch tailings are liable to be generated at the inner circumference of the center hole 74, and a disk substrate 73 with abnormal birefringence may be molded or gate cutting stroke may be varied, resulting in a defective product.
However, when the center hole 74 is punched between the projected portion 62 of the projected form gate 57 and the recessed portion 61 of the sprue bushing 56 at the timing before solidification of the molten resin P1 which is within 2 sec after the injection molding of the molten resin P1, the resin P1 before solidification would fly into the clearance between the outer circumferential surface 62a of the projected portion 62 and the inner circumferential surface 61a of the recessed portion 61, so that a flash 75 in the shape of projecting to the outside from the edge on the signal transfer surface 72 side of the center hole 74 is necessarily generated, as shown in FIG. 25. The height H1 of the flash 75 varies according to the molding conditions (the temperature of the resin P2 in the gate, and the like) of the clearance between the recessed portion 61 and the projected portion 62 shown in FIGS. 22 and 23; hitherto, the height H1 has been several tens of μm to as large as 100 μm.
The disk substrate 73 shown in FIG. 26 is one that is obtained by injection molding by a mold apparatus using a movable-side stamper according to the prior art and punching the center hole 74 from the signal transfer surface 72 side to the opposite side. In this case, there is generated a flash 75 in the shape of projecting outwards from the surface opposite to the signal transfer surface 72 of the center hole 74, and the height H1 of the flash 75 is equivalent to that shown in FIG. 25.
FIG. 27 shows a conventional disk substrate taking-out robot 81 by which the disk substrate 73 injection molded by the mold apparatus 51 is taken out of the mold apparatus 51 and transferred onto an aligning machine (not shown).
Namely, where the disk substrate 73 is injection molded by the mold apparatus 51 of the projected form gate cutting system using a conventional fixed-side stamper described referring to FIG. 22, the punching of the center hole 74 (gate cutting) is conducted by the projected form gate cutter 57 which is projected in the direction of arrow b from the movable die 53 side toward the fixed die 52, as described referring to FIG. 24. In the case of taking out the injection molded disk substrate 73 from the mold apparatus 51, the movable die 53 is opened (spaced away) from he fixed die 52 in the direction of arrow a in FIG. 22, and the sprue and gate remaining resin 73a and the disk substrate 73 are stripped off from the movable die 53 in the direction of arrow b in FIG. 22 by the ejector pin 58, the ejector 59 and the like, when the disk substrate 73 and the sprue and gate remaining resin 73a are spaced away from each other so that the disk substrate 73 is left on the side of the direction of arrow a which is the side of the movable die 53 whereas the sprue and gate remaining resin 73a is left on the side of the direction of arrow b which is the side of the fixed die 52, as shown in FIG. 27.
The robot 81, first, in FIG. 22, chucks an outer circumferential portion of the center hole 74 of the disk substrate 73, which is stripped from the movable die 53 in the direction of arrow b by the ejector 59, by sucking by a vacuum pad 82 from the side of the signal transfer surface 72, and receives the disk substrate 73 in the manner of separating the disk substrate 73 away from the movable die 53 in the direction of arrow b. Simultaneously, the sprue and gate remaining resin 73a ejected in the direction of arrow b by the ejector pin 58 is gripped by the robot 81, and the disk substrate 73 and the sprue and gate remaining resin 73a are taken out from between the fixed die 52 and the movable die 53.
Next, the disk substrate 73 is transferred to the aligning machine by the robot 81, and is aligned by fitting the center hole 74 of the disk substrate 73 onto a disk-receiving arm (not shown) of the aligning machine from the side of a reference surface 76 (described later) which is the surface opposite to the signal transfer surface 72. After the disk substrate 73 is transferred to the aligning machine, the sprue and gate remaining resin 73a is discharged from the robot 81 by spontaneous falling or by blowing air.
The disk substrate 73 injection molded as mentioned above and shown in FIGS. 25 and 26 is then subjected to lamination (coating) of a plurality of layers in the order of a recording layer, a reflective layer and a protective layer on the signal transfer layer 72, to which signals 71 have been transferred, whereby an optical disk 77 such as a CD and a DVD is completed.
In an optical disk drive device on which the optical disk 77 or the like is used, a laser beam is incident from the reference surface 76 opposite to the signal transfer surface 72, wherein writing and reading of information are conducted. According to a specification, the reference surface 76 which is a laser beam incident surface becomes a reference surface of height, a positioning center pin of a spindle motor used for driving the disk is inserted into the center hole 74 from the side of the reference surface 76, and centering is conducted by an edge 74b on the side of the reference surface 76 opposite to the signal transfer surface 72 of the center hole 74. Therefore, though the disk substrate 73 with the flash 75 generated on the side of the signal transfer surface 72, as shown in FIG. 25, can be centered without bad effects of the flash 75, the disk substrate 73 with the flash 75 generated on the side of the reference surface 76 as shown in FIG. 26 cannot be centered with high accuracy.
In the optical disk drive device, a servo mechanism with high accuracy is used in order to write and read information by focusing and reflecting a laser beam on the signals 71 constituted of fine roughness. However, the servo performance of the servo mechanism has a limitation, and, particularly, it is important to restrain eccentricity of a spirally shaped groove and the center hole 74. In recent years, the allowable value of the eccentricity of the groove and the center hole 74 has been reduced attendant on an increase in the recording density, to 100 μm in the case of CD and to 60 μm in the case of DVD.
On the other hand, the increase in the recording density is realized mainly by a decrease in the wavelength of the laser beam and an enhancement of NA (enhancement of lens magnification) of a focusing lens (objective lens). In the case of the CD which is most popular, a laser with a wavelength of 780 nm and a focusing lens with an NA of 0.45 are used, and in the case of the DVD, a laser with a wavelength of 630 nm and a focusing lens with an NA of 0.6 are used. Attendant on the enhancement of NA of the focusing lens, the disk substrate 73 which is a laser beam transmission layer is gradually reduced in thickness in order to reduce the effect of aberration. A disk substrate 73 with a thickness of 1.2 mm is used in the case of CD, and a disk substrate with a thickness of 0.6 mm is used in the case of DVD, thereby contributing to coping with camber of the optical disk.
In recent years, in order to contrive a further increase in recording density, it has been proposed to use a laser with a wavelength of 400 nm and a focusing lens with an NA of 0.85. Where the focusing lens enhanced in NA is used, it is necessary to reduce the thickness of the disk substrate 73 to about 0.1 mm; in general injection molding, however, it is difficult to mold an ultrathin disk substrate 73 which satisfies the specifications of camber and birefringence.
In view of the above, these problems have been solved by laminating a reflective layer, a dielectric layer, a recording layer and a dielectric layer, in this order opposite to the conventional order, on the signal transfer surface 72 of the disk substrate 73 to which the signals 71 constituted of fine roughness have been transferred in the conventional manner, and, finally, forming a light transmission layer of 0.1 mm in thickness.
In this case, however, the reference surface is on the opposite side as compared with the conventional disk substrate 73, and, as shown in FIGS. 25 and 26, a tapered positioning center pin 79 of a disk table 78 in the spindle motor used for driving the disk is inserted in the center hole 74 from the signal transfer surface 72 side and chucked, and centering is conducted by an edge 74a on the signal transfer surface 72 side of the center hole 74.
In this case, the disk substrate 73 shown in FIG. 26 has no flash 75 generated at the edge 74a on the signal transfer surface 72 side of the center hole 74, so that it can be centered with high accuracy. On the other hand, the disk substrate 73 having a flash 75 generated at the edge 74a on the signal transfer surface 72 side of the center hole 74 as shown in FIG. 25 cannot be centered with high accuracy.
In addition, since this system has a very small allowable value of the eccentricity of the disk substrate 73, the flash 75 generated on the signal transfer surface 72 side of the center hole 74 as shown in FIG. 25 is fatal.
Although the flash 75 can be removed by a method of cutting by a reamer or the like, the generation of waste upon removal of the flash 75 and the increase in the number of working steps lead necessarily to deterioration of yield and an increase in cost.
In order to restrain the generation of the flash 75 on the signal transfer surface 72, the stamper 55 to be fitted to the mold may be disposed not on the side of the fixed die 52 but on the side of the movable die 53 (this method is called “movable-side stamper”). Although the generation of the flash 75 on the signal transfer surface 72 can surely be obviated by this method, the movable-side stamper has such a structure that the shapes of the pits and the groove to be transferred are liable to be asymmetric. The reason is as follows. In this method, at the time of molding, after completion of cooling, the movable die 53 is opened while keeping the disk substrate 73 sucked onto the movable die 53 side, and the disk substrate 73 is taken out in the manner of stripping the stamper 55 and the disk substrate 73 by the ejector 59 and air. However, after the movable die 53 is opened, the disk substrate 73 is rapidly cooled and shrinks. Since the stamper 55 shrinks less as compared with the disk substrate 73, however, the pits and the groove are deformed. The accuracy required of the shape of the fine roughness increases as the recording density of the disk increases, and, therefore, it is difficult to mold the disk substrate 73 even by the movable-side stamper.
Even if a disk substrate to which fine roughness free of deformation has been transferred can be molded by using the movable-side stamper under a material and molding conditions substantially free of shrinkage, the generation of the flash 75 on the side of the surface 76 opposite to the signal transfer surface 72 cannot be avoided. Though the flash 75 does not have bad effects on the disk eccentricity at the time of chucking, waste might be generated through stripping of the flash 75 at the time of production, and the flash might adhere to the disk substrate 73 to thereby cause an increase in error, leading to a reduction in yield.
The present invention has been made to solve the above-mentioned problems. Accordingly, it is an object of the present invention to provide a disk substrate free of flash at the edge of a center hole, a mold apparatus optimum for injection molding the disk substrate, and a disk ejection apparatus optimum for taking out the disk substrate from the mold apparatus.